1. Field of the Invention
This invention relates to still projectors, preferably to such equipped with interchangeable lenses, and more particularly, this invention relates to a still projector provided with a compensation device acting in vertical direction compensating for the so-called Keystone-effect.
2. Technical Considerations and Prior Art
The compensation device, of which the basic design is known per se, includes means for moving a lens of a still projector perpendicular to its optical axis in a vertical direction, a lighting means, pivotal in vertical direction, about the centre of a transparent original, and means for automatically directing the lighting means toward the exit pupil of the lens.
When a transparent original is projected onto a projecting screen by a projector, the lens of which is movable only in axial direction for focusing, the image projected on the projecting screen angularly conforms, when the projection takes place according to the normal situation, i.e., when normals through the centres of the transparent original and the projected image coincide. In other projection cases, said normals form angles with each other, which results in an angularly nonconforming projection of the transparent original. In the case of vertical angle deviation from the normal case, the lateral edges of the projected image are not parallel relative to one another, and at horizontal deviation, the upper and lower edges of the image are not parallel with each other. This projection error is called Keystone-effect. Moreover, with high-speed lenses and relatively large deviation from the normal case, it is not always possible to focus the lens, so that a sufficient sharpness over the entire image plane is obtained.
When necessary or desirable, projectors can, in a known manner, be equipped with compensation devices neutralizing the Keystone-effect. A projector, equipped with such a compensation device, is provided with a lens, the angular field of which is substantially greater than that required for the image size at projection, according to the normal case. The lens is mounted, so as to be movable perpendicularly to its optical axis in at least one direction. The lighting means of the projector is rotatably mounted, with the centre of rotation in the centre of the transparent original, and coupled to a guide means disposed in the rear nodal plane of the lens. By moving the lens in parallel, it is possible, within the limits of its angular field, to correct for the Keystone-effect, and simultaneously, the lens guides the lighting means, so that it is automatically directed to the exit pupil of the lens, and the transparent original is lightened uniformly. Devices of this kind are usually provided on large film projectors for professional use, for example, as disclosed in the patent specifications U.S. Pat. No. 1,952,522 or Great Britain No. 614 473. Still projectors are very seldom equipped with such compensation devices, and if they are, the projectors are intended to be used for some special purpose, for example, for showing several originals overlapping each other, as is apparent from the disclosure of U.S. Pat. No. 3,036,496.
When showing transparent originals, local conditions seldom permit the projector and projecting screen be placed, so that the projection can take place according to the normal situation, without involving other disadvantages. For example, the projecting screen may be positioned uncomfortably for the observer, and the projector unsuitably positioned for operation. In many cases, therefore, a compromise is made by permitting a moderate Keystone-effect, in order to diminish or eliminate other disadvantages. A moderate Keystone-effect can be tolerated, when only one picture at a time is shown, as was most usually the case before the instant invention. The observer then can concentrate to this single picture, which is shown for a sufficiently long time, to enable the observer to look away from the angular errors in the picture. It has been for this reason that, heretofore, still projectors were not provided with compensation devices of the aforesaid kind. Furthermore, such devices would render the projectors larger and more expensive.
Another method of showing transparent originals, which has become increasingly usual, is to arrange for so-called multiple projection in which several projectors, usually three to seven, are used which quickly show different originals relative to one another, which are placed edge to edge according to a definite program on a projecting screen common to all originals, to thereby bring about a strong visual effect. However, the desired effect soon results in weariness and the observers become irritated when, due to the Keystone-effect, the lateral edges of the images match badly, and the deviation from the normal situation gives rise to partial blurring of the images. In view of the aforesaid difficulties in arranging a show of transparent originals, with projection according to the normal situation, it is, therefore, desirable that still projectors suitable for multiple projection are equipped with a device, which compensates for the Keystone-effect. The most frequent deviation from the normal case occurs when showing transparent originals, and the deviation most difficult to avoid is vertical deviation. In order not to render the projector unnecessarily expensive, it is, therefore, in most cases sufficient to restrict the device to compensate for vertical deviations from the normal case.